The Boy Electrician

by Alfred Powell Morgan

CHAPTER VI ELECTRICAL UNITS

The Ampere

There are certain terms used in the electrical field to distinguish various properties and qualities of the electrical current with which it is well for the young experimenter to acquaint himself.

One of the first units usually required, in order to make intelligent comparisons, is a unit of measure. The quart is the unit of measure commonly applied to liquids and is based upon the amount of space occupied by a certain volume. The pound is a unit of weight which determines a certain amount of any substance by comparing the force which gravity exerts in pulling it to the earth with the same effect of gravity on another standard "weight."

Electric current is invisible and weightless, and for these and other reasons cannot be measured by the quart or weighed by the pound. The only way that it can be measured is by means of some of the effects which it produces. Either the chemical, electro-magnetic, or the heating effects may be made the basis of a system of measurement.

The first method used to measure electric current was the chemical one.

If a current is passed through a solution of a chemical called copper sulphate (blue vitriol) by means of two copper plates, copper will be deposited on one plate and dissolved from the other. If the current is furnished by a battery the copper will be deposited on the plate connected with the zinc of the battery. If the current is allowed to flow for a short time and the two copper plates are then taken out and weighed it will be found that one plate is considerably heavier than the other.

The copper has been taken from one plate and deposited on the other by the electric currents. The amount of electric current which will deposit 1.177 grammes of copper in an hour is called an ampere. The ampere is the unit of electrical current measurement, and implies quantity or amount.

The chemical method of measuring current was at one time put to practical service in the distribution of electric current for lighting and power. Many years ago the house meters, used to measure the current, consisted of a jar containing two copper plates. The current used in the house would cause copper to deposit on one plate, and by weighing the plate the power company could determine the amount of current used, and thereby the amount of the bill. The meters nowadays make use of the magnetic effects of the current instead of the chemical, as described later on.

The Volt

For purposes of explanation the electric current may be likened to a stream of water flowing through a pipe.

If you hold your thumb over the end of a water-pipe through which water is flowing it will push your thumb away because of the pressure which the water exerts.

Electric currents also exert a pressure, only it is not called pressure in electrical parlance, but, spoken of as electromotive force or potential.

The pressure of the water enables it to pass through small openings and to overcome the resistance offered by the pipe.

Wires and other electrical conductors do not offer a perfectly free path to an electric current, but also possess a resistance. It is the potential of the electro-motive force which overcomes the resistance and pushes the current through the wire.

Advantage has been taken of the fact to fix a unit of electrical pressure called the volt. The pressure of the water in a water-pipe is measured in pounds, but the pressure of an electric current in a wire is measured by volts. The volt is the unit of electrical force which will cause a current of one ampere to flow through a resistance of one ohm.

The Ohm

The ohm is the unit of electrical resistance. The standard ohm is the resistance offered by a column of pure mercury having a section of one square millimeter and a length of 106.28 centimeters at a temperature of 0° centigrade.

The pressure which will force sufficient current through such a column of mercury to deposit 1.177 grammes of copper in one hour is a volt, and in doing so has passed a current of one ampere through a resistance of one ohm.

The units ohm, ampere, and volt, were named in honor of the three great electricians: Ohm, Ampère, and Volta.

These three units bear a very close relation to each other which is explained by Ohm’s Law.

Ohm’s Law is a simple statement of facts which it is well for the young electrician thoroughly to understand, for it might almost be said to be the basis of design of almost all electrical instruments.

It is simply this: The strength of a current equals the voltage divided by the resistance. It may be expressed in symbols by: C = E/R. Where C is the current in amperes, E is the potential in volts, and R the resistance in ohms.

By way of a simple example, we will suppose that a small telegraph sounder is connected to a battery and that the voltage of the battery is ten volts. We will further suppose that the resistance of the sounder connecting wires and the battery itself is five ohms. Knowing these two facts, it is very easy to find out how many amperes are flowing through the sounder by substituting these values in the equation as follows:

C = E/R

E = 10 volts and R = 5 ohms

therefore C = 10/5 or 2 amperes

In order to indicate fractions or very large values of the ampere, volt, and ohm, it is customary to use the following terms:

Milli-volt = 1/1000 of a volt

Mill-ampere = 1/1000 of an ampere

Kilo-volt = 1000 volts

Meg-ohm = 1,000,000 ohms

The Watt

It is no doubt perfectly plain that the water in a certain size of pipe at a pressure of 100 lbs. is more powerful than a stream of water in the same size of pipe at 25 lbs. pressure.

Likewise a current of electricity represents more power at 100 volts potential than the same current would at 25 volts. The unit of electrical power is called the watt. A watt is represented by a current of one ampere flowing through a wire at a potential of one volt.

The number of watts is found by multiplying the voltage by the amperage. In the case of the sounder and battery used as an example to explain Ohm’s Law, and where the voltage was 10 and the amperage found to be 2, the number of watts is 10 x 2, or 20 watts.

Seven hundred and forty-six watts represent one electrical horse-power. One thousand watts are called a kilo-watt.

The Coulomb

So far, none of the units have taken into consideration the element of time.

If water should be permitted to run out of a pipe into a tank until ten gallons had passed it would not be possible to tell at what rate the water was flowing by knowing that ten gallons had passed unless it were also known how long the water had been flowing. Ten gallons per minute or ten gallons per hour would indicate the rate of flow.

One ampere flowing for one second is the electrical unit of flow. This unit is called the coulomb.

One ampere flowing for one hour is called an ampere hour. The number of ampere hours is found by multiplying the current in amperes by the time in hours.

A battery may be said to have a capacity of 10 ampere hours. This means that it will deliver one ampere for 10 hours (1 ampere x 10 hours = 10 ampere hours) or 2 amperes for 5 hours (2 amperes x 5 hours = 10 ampere hours).

The same element of time enters into consideration in connection with the watt. One watt flowing for one hour is a watt hour and one kilowatt flowing for one hour is a kilo-watt hour.

The Difference between Alternating and Direct Currents

There are two distinct kinds of electric current supplied for lighting and power, one known as direct current and the other as alternating.

A direct current is one which passes in one direction only. It may be represented by a straight line, as A in Figure 88.

An alternating current is one which reverses its direction and passes first one way and then the other. It may be represented by a curved line, shown in Figure 88. It starts at zero, and gradually grows stronger and stronger. Then it commences to die away until no current is flowing. At this point it reverses and commences to flow in the opposite direction, rising gradually and then dying away again.

This is repeated a definite number of times per second; when the current rises from zero, reverses and returns to zero, it is said to pass through a cycle.

Fig. 88.—Graphic Representation of a Direct and an Alternating Current.

The part of the curved line from a to b in Figure 88 represents the first part of the current, when it is rising. From b to c represents its fall. The point at which the curved line crosses the straight line is zero. At c the current crosses the line and commences to flow in the opposite direction until it reaches d, at which point it dies away and again crosses the line to flow in its original direction and repeat the cycle.

In electrical parlance, that part of the current from a to c or from c to e is known as an alternation. From a to e is called a cycle.

The reason why alternating current is often used in place of direct current is that it can be sent over the wires for long distances more economically than direct current. This is more fully explained farther on in the chapter dealing with a step-down transformer.

The number of cycles taking place in one second is known as the frequency of the current. The usual frequency of commercial alternating currents is 60 cycles per second or 7200 alternations per minute.